Cell Biology: Structure and Function of Cells

Cell Size

Overview of Cell Size

Cells are the fundamental units of life, and their size varies greatly among different organisms. Most cells are microscopic and cannot be seen with the naked eye.

• Smallest cell: The bacterium Mycoplasma (~0.2 μm).

• Largest cell: Bird egg.

• Plant and animal cells: Typically ~10 times larger than bacterial cells.

Restrictions on Cell Size

• Cells must be large enough to contain all necessary DNA, proteins, and internal structures for survival and reproduction.

• Cells must be small enough to efficiently take up nutrients and remove wastes.

Surface Area to Volume Ratio

• Large cells have a smaller surface area to volume ratio than small cells.

• This ratio affects the cell's ability to exchange materials with its environment.

Example: A cube with a larger volume has less surface area per unit volume, making transport less efficient.

Prokaryotic Cells vs Eukaryotic Cells

Classification and Key Differences

Cells are classified into two main groups based on size and complexity: prokaryotes and eukaryotes.

• Prokaryotes: Bacteria and archaea.

• Eukaryotes: Protists, fungi, plants, and animals.

Common Features

• Both have a plasma membrane.

• Both contain chromosomes composed of DNA.

• Both have ribosomes for protein synthesis.

Key Differences

• Eukaryotes: Have a membrane-bound nucleus and membrane-bound organelles.

• Prokaryotes: Lack a nucleus and membrane-bound organelles.

Prokaryotic Cells

Structure and Features

Prokaryotic cells are simple and small, typically 1-10 μm in diameter, which is about one-tenth the size of a typical eukaryotic cell.

• No nucleus: DNA is located in the nucleoid region, which is not membrane-bound.

• Chromosome: Usually a single, circular DNA molecule.

• Ribosomes: Smaller and structurally different from eukaryotic ribosomes; targeted by certain antibiotics.

Cell Wall and Additional Structures

• Cell wall: Chemically complex, provides protection and maintains shape.

• Capsule: Some bacteria have an extra protective layer (capsule) that aids in adherence to surfaces.

• Pili: Surface projections that help in attachment to surfaces and other bacteria.

• Flagella: Longer than pili, used for motility.

Eukaryotic Cells

Structure and Features

Eukaryotic cells are found in plants, animals, fungi, and protists. They are larger and more complex than prokaryotic cells.

• Nucleus: Bound by a double membrane, contains linear chromosomes and a nucleolus (site of ribosome synthesis).

• Membrane-bound organelles: Specialized structures within the cytoplasm that perform specific functions.

Plant vs Animal Cells

• Plant cells: Have a rigid cell wall, chloroplasts (site of photosynthesis), and a large central vacuole. Lack lysosomes and centrioles.

• Animal cells: Lack a cell wall and chloroplasts, but have lysosomes and centrioles.

Example: Plasmodesmata are channels in plant cell walls that allow for exchange of materials between cells.

Plasma Membrane Structure

Composition and Function

The plasma membrane separates the cell from its environment and regulates the passage of materials.

• Phospholipid bilayer: Composed of hydrophilic heads facing outward and hydrophobic tails facing inward.

• Membrane proteins: Embedded or attached to the bilayer; hydrophilic regions face the exterior, hydrophobic regions are within the membrane.

• Permeability: Non-polar molecules (e.g., O2, CO2) pass easily; polar molecules and ions require protein channels.

Nucleus and Ribosomes

Genetic Information and Protein Synthesis

The nucleus contains most of the cell's DNA, organized as chromatin (DNA complexed with proteins). The nucleolus within the nucleus is the site of ribosome synthesis.

• Nuclear envelope: Double membrane with protein-lined pores, continuous with the endoplasmic reticulum.

• Ribosomes: Composed of large (60S) and small (40S) subunits, together forming an 80S ribosome in eukaryotes.

• Free ribosomes: Synthesize proteins for use in the cytoplasm.

• Bound ribosomes: Attached to the endoplasmic reticulum, synthesize proteins for membranes or secretion.

Endomembrane System

Components and Functions

The endomembrane system is a network of membranes involved in synthesis, storage, and export of molecules.

• Includes: Nuclear membrane, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Connected physically or via vesicles.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes membrane phospholipids and proteins for export or membranes.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Golgi Apparatus

• Receives, modifies, and ships proteins from the ER.

• Consists of stacked, non-interconnected sacs.

• Modifies glycoproteins and adds identification tags (e.g., phosphate groups).

Lysosomes

• Membrane-bound sacs containing digestive enzymes.

• Break down food, bacteria, and damaged organelles.

• Tay-Sachs disease: Caused by missing lysosomal enzyme, leading to accumulation of lipids in the brain.

Vacuoles

• Large central vacuole in plant cells stores water, nutrients, and waste; may contain pigments or poisons.

Mitochondria and Chloroplasts

Energy Conversion Organelles

Mitochondria and chloroplasts are specialized organelles for energy conversion in eukaryotic cells.

• Mitochondria: Site of cellular respiration; converts chemical energy from food into ATP.

• Structure: Double membrane, intermembrane space, and matrix (contains DNA, ribosomes, enzymes).

• Cristae: Folds in the inner membrane that increase surface area for ATP synthesis.

• Chloroplasts: Site of photosynthesis in plants; converts light energy into chemical energy (sugars).

• Structure: Double membrane, stroma (fluid with DNA, ribosomes, enzymes), thylakoids (membranous sacs), and grana (stacks of thylakoids).

Endosymbiotic Theory

• Mitochondria and chloroplasts originated as prokaryotes engulfed by ancestral eukaryotic cells.

• Both have circular DNA, prokaryote-like ribosomes, and reproduce by splitting.

Internal Skeleton: The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that provides structural support, movement, and organization within the cell.

• Microfilaments: Thin rods composed of actin; support cell shape and are involved in movement.

• Intermediate filaments: Rope-like fibers; reinforce cell shape and anchor organelles.

• Microtubules: Hollow tubes made of tubulin; shape and support the cell, serve as tracks for organelle movement, and are important in cell division.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	1-10 μm	10-100 μm
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Chromosomes	Single, circular	Multiple, linear
Organelles	None (except ribosomes)	Membrane-bound organelles present
Cell Wall	Present (complex)	Present in plants/fungi; absent in animals
Ribosomes	Smaller (70S)	Larger (80S)

Key Equations

• Surface Area of a Cube: $SA = 6a^2$

• Volume of a Cube: $V = a^3$

• Surface Area to Volume Ratio: $\text{Ratio} = \frac{6a^2}{a^3} = \frac{6}{a}$